1. Field of the Invention
The present invention relates to a television receiver having a plurality of tuners like a picture in picture (PIP, hereinafter) system.
2. Description of the Related Art
A television receiver of a PIP system has two tuners for a main picture and a sub picture and can display images of different channels simultaneously in the main picture and the sub picture.
In the conventional television receiver with a PIP function, a sub picture signal is not shown on a cathode-ray tube (CRT) when the sub picture display is off; however, a sub picture tuner is generally kept operated.
In the aforesaid television receiver, when the main picture display is executed with the sub picture display, beat interference is generated in the main picture depending on each selected channel in both pictures.
For example, let us consider a case when an UHF channel 15 is selected as a main picture and a VHF channel 6 is selected as a sub picture. Here, an example of frequencies of the television broadcast channels in Japan (12 channels in VHF band and 50 channels in UHF band, thus, 62 channels all together) will be explained. A frequency band of the channel 15 is 482-488 MHz. A frequency band of the channel 6 is 182-188 MHz and its local oscillation frequency is 242 MHz. Therefore, a harmonic wave twice the value (242.times.2=484 MHz) of the local oscillation frequency of channel 6 will interfere with a video signal 483.25 MHz of channel 15, so that a difference signal beat of 0.75 MHz is generated. Thus, beat interference occurs on the main picture.
In another example, when an UHF channel 52 (a frequency band 704-710 MHz) is selected as a main picture and a VHF channel 5 (a frequency band 176-182 MHz) is selected as a sub picture, a harmonic wave three times the value (708 MHz) of the local oscillation frequency 236 MHz of the sub picture channel 5 is generated from the tuner for the sub picture and interferes with a video signal 705.25 MHz of the UHF channel 52 of the main picture, so that stripes of a different signal beat 2.75 MHz are generated as picture interference.
Also, beat interference based on the frequency deviation (mutual frequency deviation from a specified frequency 58.75 MHz) between two video intermediate frequency outputs received by two tuners for the main picture and the sub picture, respectively, or voice beat interference based on the frequency deviation (mutual frequency deviation from a specified frequency 54.25 MHz) between two sound intermediate frequency outputs is generated.
Thus, a beat which a harmonic wave n times as large as the local oscillation frequency, a beat due to mutual interference between main and sub in an intermediate frequency output received by two tuners, or a voice beat is generated.
The aforesaid beat interference is generated as long as the tuner for the sub picture the operating, condition even if the sub picture display is off, thereby damaging the quality of the main picture.
In order to resolve the beat interference, a filter is added between a common power source of the tuners and a common control line of the tuners, and a booster or the like is added to a television signal branching device to improve isolation, so that the interference can be improved to some extent. However, it is difficult to reduce the interference completely by direct induction based on the disposition of both tuners.
Then, when a tuner is not needed for use (for example, when the sub picture is off in PIP) in the television receiver having a plurality of tuners, a method is provided for turning off the power source of the tuner as a method of improving the aforesaid mutual interference.
FIG. 4 shows an example of a television receiver having a PIP function of prior art. A television signal of a high frequency wave (including a VHF signal and an UHF signal) is supplied to an input terminal 1. The television signal is branched by a branching device 2 and fed to a main picture tuner 3 and a sub picture tuner 4, respectively. To each channel selection controlling terminal of the tuners 3 and 4, channel selection controlling signals S1 and S2 in accordance with the receiving channels of the tuners 3 and 4 are supplied to channel selecting terminals of the tuners 3 and 4 from a microcomputer 5 for channel selecting control, respectively. The receiving channels of the tuners 3 and 4 are specified by key inputting means 6. A video intermediate frequency signal from the tuner 3 is detected by a video detector 7 and supplied to the next circuit as a video output for the main picture. A video intermediate frequency signal from the tuner 4 is detected by the video detector 9 and supplied to the next circuit as a video output for the sub picture. Source voltage VDD is supplied to the tuner 3 from dc power source, while the source voltage VDD is supplied to the tuner 4 through a switch 11. The microcomputer 5 feeds the channel selection controlling signals S1 and S2 in accordance with the key inputting means 6 to the tuners 3 and 4 and indicates the channels to be received by the tuners 3 and 4, respectively. Also, the microcomputer 5 outputs a control signal S3 for making the switch 11 on or off (that is, making the sub picture tuner 4 on or off) by making the sub picture display on or off using the key inputting means 6.
Thereby, the power source of the sub picture tuner 4 is turned off by the switch 11 when the sub picture is off, so that the interference affecting the main picture can be deleted.
FIG. 5 shows another prior art of a television receiver having the PIP function. In FIG. 5, main and sub pictures are formed so as to be received by one circuit of the two tuners 3 and 4, thereby, for example, a main picture and a sub picture can be exchanged. A different point of the circuit in FIG. 5 from that in FIG. 4 is to supply dc source voltage VDD to the tuner 4 through a switch 11, while the VDD is supplied to the tuner 3 through a switch 12, and the switches 11 and 12 are turned on/off by control signals S3 and S4 from a microcomputer 5A based on the key inputting means 6, respectively. Also, a video output from a video detector 7 is fed to each of the input terminals a of switches 13 and 14, and a video output from a video detector 9 is supplied to each of the other input terminals b of the switches 13 and 14. The input terminals a and b of each switch can be controlled by signals S5 and S6. The video output for the main picture is taken out from the output terminal c of the switch 13 and the video output for the sub picture is taken out from the output terminal c of the switch 14. The input terminals a and b of the switches 13 and 14 can be selectively changed over by the control signals S5 and S6, respectively, from the microcomputer 5A based on the key inputting means 6.
Accordingly, when the sub picture is off, the power source of a sub picture tuner of the tuners 3 and 4 is off by the switches 11 or 12, so that the interference affecting the main picture can be deleted.
In the television receiver having such a plurality of tuners which can receive the same frequency band, an abnormal picture is still developed in the circuit in which a power supply is turned off in order to erase mutual interference between tuners as a fluctuation in the power source load is created when on or off is made or due to the transient response when it is on or off. If the prevention of the abnormal picture is considered, difficulties arise in designing the television receiver to not include disadvantageous costs.
As the aforesaid, in the conventional television receiver, there has been problems in which beat interference is generated in the main picture when the sub picture tuner is kept working at the time of the sub picture is off and in which it is difficult to design the television receiver and the costs of surrounding circuits rise if the fluctuation of the power source load when it is on or off and the transient response are considered in the circuit which makes the tuner power source for the sub picture off when the sub picture is off.